New three megawatt (MW) doubly fed induction generators (DFIG) under development by the owner of the present subject matter incorporate the first DFIG converters where it has become necessary to operate multiple power bridges in parallel. In order to design a power converter with low cost and high reliability, it has previously been necessary to connect insulated-gate bipolar transistor (IGBT) bridges in parallel, and balance the current sharing on parallel IGBT modules. Such a system is illustrated in FIGS. 1 and 2. In order to place the teachings of the current invention into context, a review of aspects of components for generating electricity using a wind turbine are now discussed.
Referring to FIG. 1, there is shown an exemplary embodiment of aspects of wind turbine system 100. In this embodiment, a rotor 106 includes a plurality of rotor blades 108 coupled to a rotating hub 110. The hub 110 is coupled to an optional gear box 118, which is, in turn, coupled to a generator 120. In accordance with present disclosure, generator 120 is a doubly fed induction generator (DFIG) 120.
DFIG 120 is typically coupled to stator bus 154 and a power conversion component 162 via a rotor bus 156. The stator bus 154 provides output of three-phase power from a stator (not separately illustrated) of DFIG 120 and the rotor bus 156 provides output of three-phase power from a rotor (not separately illustrated) of the DFIG 120. With particular reference to the power conversion component 162, DFIG. 120 is coupled via the rotor bus 156 to a rotor side converter 166. The rotor side converter 166 is coupled to a line side converter 168, which in turn is coupled to line side bus 188. In exemplary configurations, the rotor side converter 166 and the line side converter 168 are configured for a normal operating mode in a three-phase, two level, Pulse Width Modulation (PWM) arrangement using Insulated Gate Bipolar Transistor (IGBT) switching devices as illustrated in FIG. 2. The rotor side converter 166 and the line side converter 168 are coupled via a DC link 136 across which is the DC link capacitor 138.
The power conversion component 162 also includes a controller 174 to control the operation of the rotor side converter 166 and the line side converter 168. It should be noted that the controller 174, in typical embodiments, is configured as an interface between the power conversion component 162 and a control system 176.
In typical configurations, various line contactors and circuit breakers including, for example, grid breaker 182 may be included to isolate the various components as necessary for normal operation of DFIG 120 during connection to and disconnection from power grid 184. A system circuit breaker 178 couples the system bus 160 to transformer 180, which is connected to power grid 184 via grid breaker 182.
In operation, power generated at DFIG 120 by the rotating rotor 106 is provided via a dual path to power grid 184. The dual paths are defined by the stator bus 154 and the rotor bus 156. On the rotor bus 156 side, sinusoidal three-phase alternating current (AC) power is converted to direct current (DC) power by the power conversion component 162. The converted power from the power conversion component 162 is combined with the power from the stator of DFIG 120 to provide three-phase power having a frequency that is maintained substantially constant, for example, at a sixty Hertz AC level. The power conversion component 162 compensates or adjusts the frequency of the three-phase power from the rotor of DFIG 120 for changes.
As is known in the art, various circuit breakers and switches within the wind turbine system 100, including grid breaker 182, system breaker 178, stator sync switch 158, converter breaker 186, and line contactor 172 are configured to connect or disconnect corresponding buses, for example, when current flow is excessive and can damage the components of the wind turbine system 100 or for other operational considerations. Additional protection components (not shown) may also be provided.
It should be noted that wind turbine system 100 generates power as is known in the art and may be modified to operate in connection with different power systems, etc. It should also be recognized that aspects of wind turbine system 100 as discussed herein are merely illustrative and not limiting thereof.
In various embodiments, the power conversion component 162 receives control signals from, for example, the control system 176 via the controller 174. The control signals are based, among other things, on sensed conditions or operating characteristics of the wind turbine system 100. Typically, the control signals provide for control of the operation of the power conversion component 162. For example, feedback in the form of sensed speed of the DFIG 120 may be used to control the conversion of the output power from the rotor bus 156 to maintain a proper and balanced three-phase power supply. Other feedback from other sensors also may be used by the control system 174 to control the power conversion component 162, including, for example, stator and rotor bus voltages and current feedbacks. Using the various forms of feedback information, and for example, switching control signals, stator synchronizing switch control signals and system circuit breaker control (trip) signals may be generated in any known manner.
With reference to FIG. 2, the power converter system utilizes a DFIG converter 200 with two parallel H-bridges 202, 204 on each phase of the rotor side as shown in FIG. 2, wherein the two parallel bridges 202, 204 are coupled together with output inductors 212, 214. As is known, rotor shunt devices 216, 218 are employed to monitor current flow Irotor through their respective inductors 212, 214 as a part of the control system (not separately illustrated). The cost and reliability of such a system are a concern due to the number of extra components.
DFIGs have been used in conjunction with wind turbines for reactive power control in response to fluctuations in wind speed. In addition, some wind turbine systems have been configured to use power converters to adjust their outputs to match the grid frequency. However, such reactive techniques do not provide a method for maintaining a selected output frequency during modifications to turbine speed, for example, to increase efficiency, such as during turbine turn-down or modifications of turbine speed, for example, in response to power demands.
It is known in the art to use a single DFIG system and to modulate the power output and frequency of such a power generation unit coupled to a power grid. Using such a DFIG system, the turbine speed can be modified without disturbing the generator output frequency. These DFIG systems couple a single DFIG with both a turbine and converter such that the converter compensates for variations in the DFIG output frequency caused by changing turbines speeds. In such systems, compensation is provided by varying the excitation of the generator rotor to control the stator output frequency to match the grid frequency.
While single DFIG systems are effective at controlling output frequency, such systems generally can only provide up to about 3 megawatts of power, which is insufficient for operation at a utility scale, for example, in the range of 100 to 500 megawatts. Single DFIG systems have limited power generation capacity due to manufacturing limitations with respect to the size and rating of machine components, particularly the DFIG shaft, rotor, and slip rings. Also, these single DFIG systems are difficult to implement due to the high rating of a full scale DFIG converter, which would necessarily be 10% to 20% of the generator rating. Finally, single DFIG systems do not provide for degraded mode operation, so the failure of one system component, for example, a converter, shuts down the entire system.
In view of these known issues, it would be advantageous, therefore, to provide a system and method for controlling power output and frequency for a variable speed generator at a utility scale while reducing component size, rating, and stress.